Hadrien Rattez, Fabrizio Disidoro, Jean Sulem, Emmanouil Veveakis

Velocity stepping experiments have been performed on a simulated calcite gouge using an annular shear apparatus to investigate the effect of dissolution on the frictional properties of a carbonate fault. The tested material was put in contact with hydrochloric acid at different concentration in order to dissolve the grains. Particle size analysis shows that the small grains tend to disappear due to the chemical reaction, whereas the large grains are not much affected. The dissolution process mainly induces a decrease of the fractal number of the grain size distribution. The study of the rate-and-state parameters of the gouge, which enables to characterize the ability of the fault to generate earthquakes, shows a weakening due to the dissolution and the confining stress. This weakening implies that faults can become seismogenic after the injection in a carbonate reservoir of an acid fluid. This effect is explained by the removal of small particles, countering the increase of rugosity of the grains. Based on the experimental results, a constitutive law is suggested, in order to capture the influence of dissolution and confining stress on the frictional properties of fault. It enables to determine in which conditions of maturity and degradation of a fault and at which depth, a seismic slip can be triggered by anthropogenic activities like acid/CO2 injection.